Microwave power distribution system for an airborne radar system

ABSTRACT

An antenna system for an airborne radar system. The antenna system has a number of first antenna elements and a microwave power distribution system comprising a plurality of first T/R-units being coupled to the first antenna elements for distribution of microwave power to the plurality of first antenna elements. The microwave power distribution system has an assembly of waveguides coupled to a plurality of second antenna elements directed at an angle essentially perpendicular to the number of first antenna elements, wherein the first T/R units are coupled to the waveguides for distribution of microwave energy to the second antenna elements.

TECHNICAL FIELD

The present invention refers to an antenna system for an airborne radarsystem. The antenna system comprises a number of first antenna elementsand a microwave power distribution system comprising a number ofT/R-units arranged to distribute microwave power from a microwavegenerator to the antenna elements.

BACKGROUND

In the field of radar devices for airplanes it is known to use a dorsalunit positioned on the airplane body and extending in the longitudinaldirection of the airplane, i.e. in the direction from the fore to theaft. The dorsal unit comprises a number of side looking antenna elementspositioned along the longitudinal direction of the dorsal unit for sidelooking purposes. One problem with the dorsal arrangement is that theradar cannot see in a forward or rearward direction without additionalantenna elements being placed in the front and the rear of the dorsalunit.

The prior art document U.S. Pat. No. 5,923,302 concerns an endfire arraywith monopoles on the roof of the dorsal unit. Problems with thissolution are that it is limited in terms of antenna performance,expensive in terms of a complicated electromagnetic design process, anintricate scan control and complicated manufacturing. Furthermore, thesolution results in an undesirable upwards lobe tilt unless the groundplane is bent downwards towards the ends of the dorsal unit.

In prior art is also known to use a separate antenna in the nose of theaircraft for forward looking and an antenna inside a bulbous radomesomewhere at the aft for rearward looking. The solution to equip theseantennas with extra radar systems has the disadvantage of being costly.

Alternatively, a disadvantage with the forward and rearward lookingantennas connected to a common radar is that long high-power RF feedsmust be drawn from the radar to the forward/rearward looking antennas.This solution becomes unnecessarily heavy and it blocks the possibilityto install other, important sensors in the nose radome. Therefore, alightweight solution that utilizes the power delivered by the existingT/R-units is to be preferred.

There is thus a need for an antenna solution in a radar system providingfull coverage (360°) with no moving parts, minimized drag, minimizedweight, minimized system size, low cost, high gain and an electronicscan capability, and an overall improvement of the performance of theantenna system in a radar system with regard to forward and/or rearwardlooking abilities.

SUMMARY

The present invention refers to an antenna system for a radar system foran airplane. The antenna system comprises a dorsal unit extending in alongitudinal direction, a lateral direction and a height direction. Thedorsal unit comprises two opposing long sides extending in a heightdirection and a longitudinal direction, and two opposing short sidesextending in a lateral direction and the height direction, and an upperside opposing a bottom side each extending in the longitudinal directionand the lateral direction. The longitudinal direction coincidesessentially with the longitudinal direction of the airplane when thedorsal unit is mounted onto the airplane. The dorsal unit for a sidelooking radar system comprises a number of first antenna elementspositioned on each of the long sides of the dorsal unit and in thelongitudinal direction of the dorsal unit. The antenna system comprisesa microwave power distribution system comprising a number of firsttransmit-receive units (hereinafter called T/R-units) positioned insidethe dorsal unit and arranged to feed microwave power to and from thefirst antenna elements. The power distribution system is arranged todistribute microwave power.

The present invention is characterized in that the microwave powerdistribution system comprises an assembly of polarized waveguidesmounted on top and/or on the bottom of the dorsal unit and/or inside thedorsal unit. The antenna system also comprises antenna devices mountedin connection to the short sides and extending in the height direction.The antenna devices comprise a number of second antenna elements beingfed by the power generator via the power distribution system. Theantenna devices are connected to the waveguides so that the microwavepower supplied by the first T/R-units can be distributed in such a waythat an azimuthal scan can be performed by the radar system in theforward and/or in the rearward sectors. The scan is made by controllingthe phases of the microwave power between the antenna elements bycontrolling the first T/R-units in a manner known from prior art. Thepurpose of the invention is thus to allow scanning of a fore and/or aftlobe without using an antenna in the nose of the airplane and a radomeat the tail of the same, or to use the also less satisfactory solutionof the above described end fire solution described in U.S. Pat. No.5,923,302.

One benefit of the invention is that the waveguide assembly can bedesigned and manufactured at a low cost. Further advantages are that itless expensive and more lightweight than the nose and/or aft antennaknown from prior art. A further advantage with the waveguide assembly isthat the integration into the aircraft becomes simpler to perform.

Furthermore, the following advantages are shared with the antenna systemdescribed in U.S. Pat. No. 5,923,302, namely, that the weight does notadd significantly to the dorsal unit weight, and that the waveguideassembly can be mounted onto the dorsal unit without major re-design ofan existing dorsal unit. Hence, the waveguides and the antenna devicesform a collection of parts that may easily be mounted in situ directlyonto the existing dorsal unit and inter-connected to each other andconnected to the already existing devices, for example the firstT/R-units. The first T/R-units may be coupled to the assembly ofwaveguides by connecting all first T/R-units to a dedicated waveguideand to equip adjacent waveguides with apertures for allowing theelectromagnetic signal in the dedicated waveguide to the remainingwaveguides. In an alternative embodiment one T/R-unit is coupled to oneof the waveguides and the number of waveguides and T/R-units are thusthe same. In a yet further embodiment, a few T/R-units, say N_(T/R) arecoupled to each of the N_(FWG) waveguides so that N_(T/R) multipliedwith N_(FWG) approximately equals N_(T/R,TOT). Since N_(FWG) is equal toN_(AD), the number of antenna devices, this approximate relation couldalso be expressed as: N_(T/R)=N_(T/R,TOT)/N_(AD).

However, the present invention has the following advantages over thedevice described in U.S. Pat. No. 5,923,302; it is inexpensive in termsof the electromagnetic design process, it does not need an intricatescan control, and does not need complicated manufacturing. Furthermore,the present invention does not result in an undesirable upwards lobetilt. Yet furthermore, the present invention may be designed for abetter and more controllable antenna performance in terms of lobe widthsand scannability.

The assembly may have a planar extension in the lateral direction butmay also have a somewhat dome shaped or curved cross-section, but mayalso be arranged in a staggered manner, i.e. in a zigzag pattern where anumber of waveguides being partly or fully on top of other waveguides.The planar assembly of waveguides is advantageous since it is easy tomanufacture.

In one embodiment the second antenna elements are connected to secondT/R-units being arranged to be controlled in such a way that themicrowave power supplied by the first T/R-units can be distributed bythe antenna system in such a way that an elevation scan is performed bythe radar system in a direction out from one of the short sides or boththe short sides, i.e. in the forward direction and/or the aft directionof an airplane when the dorsal unit is mounted onto the airplane.

The antenna system according to the invention may thus be used for a360° azimuthal scan in a plane described by the lateral direction andthe longitudinal direction by use of a control unit for controllingphase and amplitude of each of the first T/R-units to feed microwaveenergy via a switch to either each of the first starboard antennaelements or to each of the port antenna elements, and by use of acontrol unit for controlling phase and amplitude of each of N_(FWG)subsets of N_(T/R) T/R-units to feed one of the N_(FWG) waveguides. TheT/R-units may thus comprise a switch device controlled by the controlunit for controlling the feed of microwave energy directly to the firstantenna elements or indirectly to the second antenna elements dependenton the direction of the scan. The first antenna elements are used foressentially a lateral scan on both sides of the dorsal unit and thesecond antenna elements are used for forward and rearward scan. Theantenna system may also perform an elevation scan by controlling thesecond T/R-units to feed microwave energy to the second antenna elementswith a phase increment in the height direction.

The stated and further advantages and embodiments will become apparentin the detailed description of the invention below.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given herein below in the accompanying drawingswhich are given by way of illustration only, and thus are not limited tothe present invention and wherein:

FIG. 1 schematically shows a microwave power distribution systemaccording to a first embodiment of the invention where an assembly ofhorizontally polarized rectangular waveguides is placed above a dorsalunit.

FIG. 2 schematically shows a microwave power distribution systemaccording to a second embodiment of the invention where an assembly ofvertically polarized rectangular waveguides is placed above a dorsalunit;

FIG. 3 schematically shows a top view of an assembly of rectangularwaveguides according to FIG. 1 or 2 with different lengths;

FIG. 4 schematically shows a front (or aft) view of an assembly ofhorizontally polarized rectangular waveguides according to FIG. 1 beingfed with electromagnetic power;

FIG. 5 schematically shows a front (or aft) view of an assembly ofvertically polarized ridged waveguides according to FIG. 2 being fedwith electromagnetic power;

FIG. 6 schematically shows a top view of an antenna system comprising anassembly of rectangular waveguides according to FIG. 1 or 2 where allfirst T/R-units feed a single waveguide, and a power distributionpattern in the waveguide assembly;

FIG. 7 schematically shows a distribution arrangement for an assembly ofwaveguides according to FIG. 7 comprising apertures in the side wallsbetween the waveguides;

FIG. 8 schematically shows a top view of an antenna system comprising anassembly of rectangular waveguides according to FIG. 1 or 2 where thefirst T/R-units feed all waveguides, and wherein;

FIG. 9 schematically shows a front view of an antenna system accordingto FIG. 6 or 8.

DETAILED DESCRIPTION

FIG. 1 schematically shows an antenna system 1 for an airborne radarsystem according to a first embodiment of the invention. The antennasystem 1 comprises a number of first antenna elements 2 and a microwavepower distribution system 3 comprising a number of first T/R-units 4arranged to distribute microwave power from a microwave generator 5 tothe first antenna elements 2. The microwave power distribution system 3in FIG. 1 comprises a planar assembly of polarized waveguides 6 coupledto a number of second antenna elements 7 directed at an angleessentially perpendicular to the number of first antenna elements 2. Thefirst T/R-units 4 are arranged for distribution of microwave energydirectly to and from the first antenna elements and indirectly to andfrom the second antenna elements 2, 7 via the assembly of waveguides 6.

The antenna system comprises a dorsal unit 8 having two opposing longsides 9 extending in a height direction Z and a longitudinal directionX, and two opposing short sides 10 extending in a lateral direction Yand the height direction Z, and an upper side 11 opposing a bottom side12 each extending in the longitudinal direction X and the lateraldirection Y. The directions X, Y and Z are only mentioned in order tofacilitate the description and understanding of the invention and are inFIG. 1 depicted as an orthogonal system. It should be noted that thedorsal unit does not have to be a rectangular box, but may comprisesides having a non-planar extension. For example, the upper side 11 mayhave a somewhat dome shaped or curved cross-section taken in the lateraldirection. The waveguides 6 may then follow the shape of the upper side11 or may be arranged along a different contour.

In FIG. 1 the assembly of polarized waveguides 6 is mounted on top ofthe dorsal unit 8, i.e. on the upper side 11 of the dorsal unit 8. Theassembly of waveguides 6 may alternatively be positioned at the bottomside 12 of the dorsal unit 8 or within the dorsal unit 8. In FIG. 1 thedorsal unit 8 is mounted onto the top of an airplane 13, so that thelongitudinal direction of the airplane 13 coincides with thelongitudinal direction of the dorsal unit. The airplane 13 is left outin the remaining drawings 2-9 in order to minimize the number offeatures in the drawings. However, the dorsal unit 8 is intended to bemounted onto a device moving at high speed, particularly in the air. Thehigh speed feature is of importance since it puts high demands on theantenna system with regard to aero dynamical features such asaerodynamic drag as well as temperature and wear due to, for examplerain and sand erosion.

In FIG. 1 the first antenna elements 2 are positioned at leastlongitudinally on each of the long sides 9 of the dorsal unit 8 and thesecond antenna elements 7 are positioned in connection to one of theshort sides 10 or both the short sides 10. FIG. 1 shows antenna devices18 mounted onto both the short sides 10 of the dorsal unit 8. Theantenna devices 18 comprise the second antenna elements 7 which areconnected to the waveguides 6. The antenna devices 18 will be describedfurther below.

In FIG. 1 the first T/R-units 4 are positioned within the dorsal unit 8,but they may be positioned at a location outside the dorsal unit 8, forexample inside the airplane 13. The first T/R-units 4 are often referredto as T/R-modules and they serve the purpose of feeding RF signals fromthe microwave generator 5 to the antenna elements 2, 7 during atransmission period and receiving RF signals from the antenna elements2, 7 while switching off the energy feed during a listening period.During a first transmission period the first T/R-units 4 feeds RFsignals from the power source to the first antenna elements 2 via forexample a galvanic coupling or by use of a connect-less electromagneticcoupling.

During a second transmission period the first T/R-units 4 feed energyfrom the power source to the waveguides 6 which in turn feeds the RFsignals to the second antenna elements 7 in the fore or in the aft. Theenergy feed from the first T/R-units 4 to the waveguides 6 may be doneby any suitable means, for example by use of a galvanic conductor, i.e.a flexible cable or the like, connecting a T/R-unit 4 with atransforming device for transforming the electrical signal into anelectromagnetic microwave signal. The microwave signal is fed by thetransforming device into the waveguides where the microwave signalpropagates in a known manner. The energy from the T/R-units 4 may alsobe fed to the waveguides by means of a connect-less electromagneticcoupling

During the listening periods the antenna elements 2, 7 receive returningelectromagnetic power previously sent out having been reflected from anobject, for example a target. During the listening period the microwavepower distribution system 3 comprises means for feeding the returningmicrowave energy to receivers for signal processing in the radar system.With regard to the second antenna elements 7, the waveguides 6 areconstructed to feed the returning microwave energy directly to thereceivers or to a converting device converting the electromagneticsignals in the waveguides 6 into electric signals in cables for furtherfeeding the microwave energy to the receivers. It should be noted that ascanning period may comprise a number of transmission and receivingperiods.

In FIG. 1, the distribution system 3 comprises an assembly ofhorizontally polarized rectangular waveguides 6 according to the firstembodiment of the invention. The waveguides 6 are not limited to arectangular cross-section, but may have any geometric cross-sectionsuitable for guiding microwaves, for example round, oval, or ridged.However, the following description refers to rectangular waveguides inorder to conform to the drawings, but it should be understood that anykind of geometrical shape is possible to replace the rectangularcross-section.

Furthermore, the waveguides are not limited to horizontally polarizedwaveguides 6, but may in a second embodiment, shown in FIG. 2, be in theform of vertically polarized rectangular waveguides. The invention isnot limited to the two embodiments, but they pose mere examples. Thewaveguides may thus be circularly polarized or polarized in any othersuitable way.

FIG. 3 schematically shows a top view of an assembly of rectangularwaveguides 6 according to FIG. 1 or 2 with different lengths. Theadvantage of this embodiment is that the weight of the waveguide system6 is lessened compared to equally long waveguides. However, thewaveguides 6 may in another embodiment be equally long or essentiallyequally long, see for example any of the FIG. 1, 2, 6, 8 or 9.

FIG. 4 schematically shows a front (or aft) view of an assembly ofhorizontally polarized rectangular waveguides according to FIG. 1 beingfed with electromagnetic power from the microwave power generator 5. InFIG. 4 the waveguides 6 are fed the microwave signal by use of atransforming device comprising a probe 14 being either magnetic,electric or adapted to transmit/transform energy in any other suitableway.

FIG. 5 schematically shows a front (or aft) view of an assembly ofvertically polarized ridged waveguides 6 according to FIG. 2 being fedwith electromagnetic power by a probe 14 from the microwave generator.As been stated above, a vast amount of waveguide feeding techniques areknown from prior art which may be applied on the invention.

The feeding of energy to the waveguides 6 need to be controlled in orderto control the phase shifts in the waveguides 6 in order to direct theenergy in the front or aft direction of the airplane. Hence, the phaseincrement of the T/R-units needs to be set in order to have aconstructive adding of energy in the desired direction. Therefore, acontrol device (not shown) is comprised in the antenna system forcontrol of the first and the second antenna elements 2, 7.

FIG. 6 schematically shows a top view of an antenna system 1 comprisingan assembly of rectangular waveguides 6 according to FIG. 1 or 2 whereall first T/R-units 4 feed a single dedicated waveguide 6 a. Themicrowave power is distributed to the adjacent waveguides 6 b viaapertures (not shown). In FIG. 6 a number of arrows are shown depictinga power distribution pattern in the waveguide assembly. The waves aredistributed from the dedicated waveguide 6 a in the lateral direction Ytowards the most peripheral waveguides. The signal propagation isintended to be in the longitudinal direction of the waveguide assembly,i.e. in the direction from end to end, and is utilized at the endsections only.

In FIG. 6 the central waveguide 6 a is the dedicated waveguide, but anyother waveguide 6 in the waveguide assembly may be designated to be thededicated waveguide 6 a being fed by all first T/R-units 4. In FIG. 6 isshown that the distribution system 3 comprises two linear assemblies ofphase shift devices 16 arranged at each end of the waveguide assembly 6.One phase shift device 16 is positioned at each end of each waveguide 6.The phase shift devices 16 may be of any type known from prior art, forexample ferrite phase shift devices. The phase shift devices 16 may bemounted onto the end of the waveguide 6 or may be inserted into an endpart of the waveguide 6.

The advantages of the embodiment shown in FIG. 6 are a highly modularand low-cost design. For instance, only one feed transition, for examplethe above described probe, between the waveguide 6 assembly and each ofthe first T/R-units 4 needs to be designed. It does not offer thepossibility of scanning of the fore/aft lobe without using a linearassembly of additional phase shifters at the waveguide ends because thephase in one waveguide will be determined by the phases of theneighboring feed waveguide. However, the embodiment has the advantagethat the entire waveguide assembly 6 may be manufactured separately fromthe dorsal unit 8 and may then easily be mounted onto an alreadyexisting dorsal unit and connected to the already existing firstT/R-units 4 by simple means.

In FIG. 6 is schematically shown lobes 17 in an azimuthal scan in theX-Y-plane. The lobes extend essentially in the forward direction X andscanning is performed in the lateral direction Y.

The antenna devices according to the invention allows for an azimuthalscan in the X-Y-plane by use of phase shift devices 16 for controllingthe second antenna elements. The lobes extend essentially in the forwarddirection X and scanning is performed in the lateral direction Y. Asbefore, the second antenna elements are being fed microwave power viathe waveguides 6 and the control device controls the first T/R-units 4for the fore or aft direction. The azimuthal scan is then created by useof the control device for controlling the phase shift devices 16 so thatthe energy is directed for scanning in the X-Y-direction by the secondantenna elements 7. The second antenna elements 7 are arranged to coverthe forward sector, and in appropriate cases the aft sector, whichcannot be scanned by the first antenna elements.

The first antenna elements 2 may be used to scan a sector being 2 timesan angle α (2×α) and the second antenna elements 7 may be used to scan asector being 2×(90°-α). In FIG. 6, the angle α refers to an anglebetween a normal N extending in the lateral direction Y, i.e. in adirection being essentially perpendicular to the longitudinal directionX of the dorsal unit 8, and a line L. The angle α range between thenormal N and a tangent T in the longitudinal direction X. The antennasystem 1 may thus be used to scan 360° in the X-Y-plane. It can bementioned as an example that if the first antenna elements cover asector of 120° i.e. 2 times 60° on each side of the dorsal unit 8 andthe second antenna elements cover a sector of 60°, i.e. 2 times 30° inboth the fore and aft direction.

FIG. 7 schematically shows a distribution arrangement for an assembly ofwaveguides according to FIG. 6 comprising apertures 15 in the side wallsbetween the waveguides 6 for lateral distribution of the microwavepower. The apertures 15 may be slots, or any other suitable throughhole.

FIG. 8 schematically shows a top view of an antenna system comprising anassembly of rectangular waveguides according to FIG. 1 or 2 where thefirst T/R-units 4 feed all waveguides 6. Here, a subset of (i.e. numberof) the first T/R-units 4 are connected to each waveguide. All of thewaveguides 6, however, are fed by the first T/R-units 4. The fact thatthe feed points of a waveguide must obey certain phase relationships forefficient propagation does not prohibit that the phases between thewaveguides 6 can be given arbitrary values. Hence, fore and/or aftscanning is possible without extra phase shift devices 16. This solutionavoids the costs and weight associated with phase shift devices 16.However, the phases of the first T/R-units 4 have to be flexiblycontrolled by the control device in order to be able to control thedirection of propagation of the microwave signal in the cluster ofwaveguides 6. The control device therefore controls the first T/R-units4 according to a selected algorithm giving the control of the directionof propagation.

The waveguide assembly 6 may be manufactured separately in the samemanner as the waveguide assembly described in connection to FIG. 6. Onedifference however between the two embodiments described in connectionto FIGS. 6 and 8 respectively is that the embodiment described inconnection to FIG. 8 has to have feed transition to all waveguides, forexample by the above described probe. However, since the phase shiftdevices 16 in FIG. 6 are not necessary, the embodiment shown in FIG. 8also has the advantages of being of a highly modular and low-costdesign.

Also in the embodiment described in FIG. 8, the first and second antennaelements 2, 7 are positioned so that the antenna system 1 can becontrolled to cover a 360° azimuthal scan by alternating between thefirst antenna elements 2 and the second antenna elements 7 as describedin connection to FIG. 6.

It should be noted that each of the first T/R-units 4 are directlycoupled to the side looking first antenna elements 4 on each long sideof the dorsal unit 8, but that the first T/R-units 4 are indirectlycoupled to the second antenna elements 7 via the waveguides 6. Since atleast a number of the first T/R-units 4 are switched to a number of thewaveguides 6, the phases between the first T/R-units may be controlledso that the common signal from the first T/R-units are fed in the foreor aft direction in the waveguides 6. Hence, the first T/R-switches maybe controlled so that the antenna system may perform a scan on all sidesof the dorsal unit 8.

FIG. 9 schematically shows a front view of an antenna system accordingto FIG. 6 or 8. The antenna system 1 comprises the antenna devices 18mounted in connection to one of the short sides 10 or both the shortsides 10 and extending in the height direction Z. The antenna devices 18are preferably positioned essentially parallel to each other with aselected distance D1 between them. The selected distance D1 may bedecided dependent on the desired performance of the antenna system 1 andon minimizing the aerodynamic drag. It should be noted that it is thecenter-to-center distance that relates to the desired performance andthat the distance D1 in relation to the center-to-center distance thatrelates to drag. The second antenna elements 7 are comprised in theantenna devices 18 and are preferably positioned in each of the antennadevices 18 in a row, i.e. in a series after each other in the heightdirection Z.

The antenna devices 18 may be mounted directly onto the short side(s) 10or may be mounted to the dorsal unit via brackets 19. The antennadevices may also be interconnected via brackets 19 such that the antennadevices form a separate unit easily mounted onto an already existingdorsal unit. The antenna devices are connected to the waveguides by anyknown means, for example by contact-less connector means or galvanicconnector means. The number of antenna devices 18 is correlated to thenumber of waveguides in such a way that there is one antenna deviceconnected to each waveguide 6 One advantage of using the antenna devices18 is that the effective antenna aperture area is increased at the sametime as the aero dynamic drag is kept to a minimum. The increasedeffective antenna aperture area gives the possibilities of increasedgain and thus the possibility of increased gain and thus to create morenarrow lobes 17 for better detection of targets.

Furthermore, the antenna devices 18 are connected to the waveguides 6 sothat the microwave power supplied by the first T/R-units 4 can bedistributed by the antenna system in such a way that an azimuthal scanaccording to the above is performed by the radar system in a directionout from one of the short sides 10 or both the short sides 10, i.e. inthe forward direction and/or the aft direction of an airplane when thedorsal unit is mounted onto an airplane.

In a further embodiment the second antenna elements 7 are connected tosecond T/R-units 20 positioned between the waveguides 6 and the secondantenna elements 7. The second T/R-units 20 are arranged to becontrolled by the control unit. The microwave power supplied by thefirst T/R-units 4 is fed to the second T/R-units 20 via the waveguides6. The second T/R-units 20 are controlled by the control unit in such away that the phase increment between the second antenna elements 7 givesan elevation scan in a direction out from one or both the short sides10, i.e. in the forward direction and/or the rearward direction of anairplane when the dorsal unit is mounted onto an airplane. The antennasystem 1 may thus use the first T/R-units 4 for an azimuthal scan andthe second T/R-units for an elevation scan.

The above described scans are made by controlling phases in differentantenna elements by the control of the first- and/or the secondT/R-units 20 in a manner known from prior art and will not be explainedfurther.

The antenna devices 18 may be realized in a number of different ways.For example, each antenna device 18 comprises a layered structurecomprising in the lateral direction an electrically conducting layer 21onto a non-conducting 22 layer positioned adjacent a number of secondantenna elements 7 and on the other side of the antenna elements 7 asecond non-conducting layer 23 covered with a conductive layer 24. Thesize of the antenna devices 18 is dependent on the intended use of theantenna system, i.e. the intended use of the radar system that comprisesthe antenna system.

Below is an example of an antenna device suitable for an airborne S-bandradar: The measurements are 10 mm times 100 mm times the height whichmay be less than, equal to or greater than the height of the dorsalunit. The antenna devices are separated by a selected distance D1=70-80mm depending on a number of parameters, for example the wavelength ofthe microwave transmitted. The separation therefore has to be calculatedwith regard to these parameters.

As can be seen in FIG. 8, the antenna devices 18 form an assembly ofantenna devices 18 forming an antenna. One benefit of using such thinantenna devices 18 in the proposed manner is that the antenna may extendoutside the dorsal unit in the lateral direction without significantincrease of aerodynamic drag. The possibility to extend thecross-section of the antenna system in the forward and/or aft directionis a major benefit of the invention since the more antenna devices andthe wider the antenna system is in the lateral direction, the narrowercan the lobe be formed.

Further advantages of the invention are that a dorsal fin assembly isthin, light and requires no moving parts, and thus advantageouslyreplaces the previously known AWACS rotodome type antenna.

The invention is not limited to the above described embodiments, but maybe varied within the scope of the appended claims. For example, theembodiments described in connection to FIGS. 6 and 8 may be combined sothat a number of waveguides are fed microwave power by a number of thefirst T/R-units and where the remaining waveguides having not been feddirectly by the first T/R-units are fed by distribution via apertures inthe walls of the waveguides. However, all embodiments have the advantagethat a part of the power from each T/R-module to a series of waveguidesin, above or under the dorsal unit, the summed RF-power can betransmitted to either end (fore/aft) of the dorsal unit withoutsignificant loss of effect by use of an easily manufactured andassembled construction comprising a collection of parts.

1-17. (canceled)
 18. An antenna system for an airborne radar system, theantenna system comprising: a plurality of first antenna elements; and amicrowave power distribution system comprising a plurality of firsttransmit/receive (T/R) units being coupled to the first antenna elementsfor distribution of microwave power to the plurality of first antennaelements, wherein the microwave power distribution system comprises anassembly of waveguides coupled to a plurality of second antenna elementsdirected at an angle essentially perpendicular to the plurality of firstantenna elements and wherein the plurality of first TIR units arecoupled to the waveguides for distribution of microwave energy to theplurality of second antenna elements.
 19. The antenna system accordingto claim 18, the antenna system further comprising a dorsal unit havingtwo opposing long sides extending in a height direction (Z) and alongitudinal direction (X), and two opposing short sides extending in alateral direction (Y); the height direction (Z), and an upper sideopposing a bottom side each extending in the longitudinal direction (X)and the lateral direction (Y), wherein the assembly of waveguides aremounted on the upper side and/or on the bottom side of the dorsal unitand/or inside the dorsal unit.
 20. The antenna system according to claim19, wherein the first antenna elements are positioned at leastlongitudinally (X) on each of the long sides of the dorsal unit and thatthe plurality of second antenna elements are positioned and coupled toone of the short sides or both the short sides.
 21. The antenna systemaccording to claim 19, wherein the antenna system comprises antennadevices coupled to the waveguides and extending in the height direction(Z), wherein the plurality of second antenna elements are in the antennadevices.
 22. The antenna system according to claim 21, wherein theantenna devices are coupled to one or both of the short sides and extendin the height direction (Z), wherein the plurality of second antennaelements are in the antenna devices.
 23. The antenna system according toclaim 21, wherein the antenna devices are positioned essentiallyparallel to each other.
 24. The antenna system according to claim 21,wherein the antenna devices are coupled to the waveguides so that themicrowave power supplied by the plurality of first T/R units can bedistributed by the antenna system such that an azimuthal scan isperformed by the radar system in a direction out from one of or both ofthe short sides in the forward direction and/or the rearward directionof an airplane when the dorsal unit is mounted onto an airplane.
 25. Theantenna system according to claim 21, wherein the plurality of secondantenna elements are positioned in the antenna devices in series in theheight direction.
 26. The antenna system according to claim 25, whereinthe plurality of second antenna elements are coupled to a plurality ofsecond T/R units being arranged to be controlled in such a way that themicrowave power supplied by the plurality of first T/R units can bedistributed by the antenna system in such a way that an elevation scanis performed by the radar system in a direction out from one of theshort sides or both the short sides in the forward direction and/or theaft direction of an airplane when the dorsal unit is mounted onto theairplane.
 27. The antenna system according to claim 18, wherein theplurality of first antenna elements are coupled to the plurality offirst T/R-units such that the microwave power supplied by the pluralityof first T/R units can be distributed by the antenna system such that anazimuthal scan is performed by the radar system in a direction out fromone or both of the long sides in the lateral direction (Y) of anairplane when the dorsal unit is mounted onto the airplane.
 28. Theantenna system according to claim 18, wherein the antenna systemcomprises a control device for controlling the plurality of firstT/R-units and thereby the phase shifts of the microwave power betweenthe plurality of the first and the second antenna elements.
 29. Theantenna system according to claim 18, wherein one dedicated waveguide inthe microwave power distribution system is coupled to all the pluralityof first T/R units, wherein the waveguides comprise apertureselectromagnetically coupling adjacent waveguides such that the microwavepower is distributed from the dedicated waveguide to the remainingwaveguides in the microwave power distribution system.
 30. The antennasystem according to claim 29, wherein the apertures are positioned inthe walls of the waveguides for lateral distribution of the microwavepower.
 31. The antenna system according to claim 29, wherein themicrowave power distribution system comprises phase shift devicespositioned at each end of each waveguide for phase shifting of themicrowave power in each waveguide.
 32. The antenna system according toclaim 18, wherein each waveguide in the distribution system is coupledto at least one of the plurality of first T/R units.
 33. The antennasystem according to claim 18, wherein the plurality of the first and thesecond antenna elements are positioned such that the antenna system canbe controlled to cover a 360° azimuthal scan by alternating between theplurality of first antenna elements and the plurality of second antennaelements.
 34. The antenna system according to claim 18, wherein thewaveguides are laterally polarized or polarized in the height direction.